Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
For example, some applications require the use of ions with low kinetic energy (i.e. a few electron volts) because the material being processed is sensitive to damage. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy. Still other applications, such as reactive ion beam etching, require precise control of the ion energy.
Some applications require direct exposure of the material being processed to a high density plasma. One such application is generating ion-activated chemical reactions. Other such applications include etching of and depositing material into high aspect ratio structures. Other applications require neutral activated gases containing atoms and activated molecules while the material being processed is shielded from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements.
Various plasma sources can generate plasmas in numerous ways including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into a plasma. Induction coils are typically used for inducing current into the plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave passing window into a discharge chamber containing a gas. Microwave discharges can be used to support a wide range of discharge conditions, including highly ionized electron cyclotron resonant (ECR) plasmas.
Compared with microwave or other types of RF plasma sources, a toroidal plasma source has advantages in low electric field, low plasma chamber erosion, compactness, and cost effectiveness. The toroidal plasma source operates with a low electric field and inherently eliminates current-terminating electrodes and the associated cathode potential drop. The lower plasma chamber erosion allows toroidal plasma sources to operate at higher power densities than other types of plasma sources. In addition, the use of high permeability magnetic cores couples electromagnetic energy to plasma efficiently, allowing the toroidal plasma source to operate at relatively low RF frequencies while lowering power supply costs. Toroidal plasma sources have been used to produce chemically reactive atomic gases including fluorine, oxygen, hydrogen, nitrogen, etc. for processing semiconductor wafers, flat panel displays, and various materials.